The present invention relates to scattering cells and, in particular, to cells designed to measure both relatively small scattering angles as well as large scattering angles.
Particles disposed in a medium having a different index of refraction than the particles can scatter light. In general, the intensity of the scattered light is a function of the wave length of the incident light, the size and concentration of the scattering particles, as well as the volume of the sample being irradiated. The scattering sample can transmit a direct and a scattered beam whose intensity ratio can be related to the Rayleigh factor to obtain an indication of molecular weight.
A well known method of analyzing the scattered intensity of light is by a Zimm plot. By extrapolating this plot to zero concentration at zero scattering angle, an estimate can be made of the reciprocal of molecular weight. In addition, various slopes of the Zimm plot relate to the redius of gyration of the scattering particles. In particular the second virial coefficient of the Zimm plot can be related to the non-ideality of the scattering particles, thereby characterizing the particles by their interaction with the medium and different surrounding particles. For certain scattering particles, the slopes of the Zimm plot may be linear out to very large scattering angles. Therefore it is desirable under these circumstances to obtain a plot extending from very small to very large angles.
In the past, equipment has been designed to measure very small scattering angles but has been unable to simultaneously obtain information regarding large scattering angles. It is advantageous to obtain such simultaneous measurements since the measurements are known to be definitely related to the same type of particles so that the validity of the data is confirmed.
Another class of useful data can be obtained from observing the frequency bandwidth or Doppler shifts caused by particle motion. This effect tends to broaden the line width of the spectrum transmitted from the scattering sample. By conventional spectrum analysis or by using autocorrelation techniques, these frequency effects can be evaluated to allow derivation of the diffusion coefficient of the particles. This coefficient bears a relation to the particles' size and shape and molecular weight. The diffusion coefficient measured in this fashion can vary as a function of the scattering angle. The manner in which this diffusion coefficient changes signifies the nature and number of the degrees of freedom of the particles. For very small scattering angles, the diffusion coefficient is related primarily to the translational degrees of freedom. Such information is very valuable for studying anisotropic particles. Other types of degrees of freedom may be revealed only at relatively large angles.
It is also known to analyze particles in a medium by subjecting them to size exclusion chromatography, for example, gel permeation chromatography and high pressure liquid chromatography. It is desirable to apply such methods contemporaneously with a light scattering test so the data is reliably correlated.
A practical problem with obtaining accurate scattering measurements is accounting for extraneous particles in the medium. The clarification required to exclude such extraneous particles can be time consuming and expensive. An advance has been achieved with the advent of highly focused lasers which concentrate coherent light within an extremely small volume. The probability therefore of an extraneous particle entering within this small volume becomes rather small and its presence is obvious. Another practical problem is designing a sample cell so that light passing within its windows does not reflect back into the scattering sample. Such reflections can cause an undesirable illumination of the sample resulting in a background noise that degrades the accuracy of the measurement, expecially at very low angles.
One known technique (U.S. Pat. No. 3,843,268) for overcoming the internal reflections within the window of a sample cell involves using relatively thick windows. These thick windows provide a relatively long optical path so that light internally reflected by the window has less of a tendency to return to the sample volume. Unfortunately the design of the exit windows in U.S. Pat. No. 3,843,268 permits 180.degree. back reflections at the air/glass interface allowing incident light to reenter the scattering volume. Thus the scattering volume has incident beams in opposite direction with the reflected beam having an intensity of about 4% of the main beam. Another disadvantage with this type of system is that its use of relatively thick windows prevents measurements at relatively high scattering angles. Also the system does not attempt to increase the angular resolution for measurements at very low scattering angles.
Accordingly, there is a need for a scattering cell which is able to measure contemporaneously light scattered at relatively low and relatively large scattering angles and to perform these measurements with improved resolution.